Pristine graphene - copper (II) oxide nano catalyst: A novel and green

Research Article pubs.acs.org/journal/ascecg

Pristine Graphene−Copper(II) Oxide Nanocatalyst: A Novel and Green Approach in CuAAC Reactions Tripti Vats,‡ Rituporn Gogoi,‡ Pankaj Gaur, Anuradha Sharma, Subrata Ghosh,* and Prem Felix Siril* School of Basic Sciences, Indian Institute of Technology Mandi, Mandi-175005, Himachal Pradesh, India S Supporting Information *

ABSTRACT: Pristine graphene, as the name suggests is the closest to graphite structurally among all the forms of graphene that are synthesized using different methods. High electronic conductivity, large surface area, and absence of defects make this perfect two-dimensional arrangement of sp2 hybridized carbon atoms a perfect support material for metal or metal oxide nanoparticles. We have introduced a quick and green route to synthesize CuO nanocomposites having pristine graphene as a support material by microwave assisted hydrothermal reaction. The nanocomposite exhibited very high catalytic activity in copper catalyzed azide-alkyne cycloaddition (CuAAC) reactions compared with reduced graphene oxide (RGO)-CuO nanocomposite and CuO nanoparticles. The presence of pristine graphene in the nanocomposite increases the catalytic activity due to its better conductivity and ability to adsorb reactants through π−π interaction than RGO. The pristine graphene-CuO nanocomposite showed very good recyclability with much less leaching of the metal from it. The CuAAC reactions could be completed in a short duration (1 h), at low reaction temperature (30 °C), using water as a “green” solvent with a small amount of the pristine graphene-CuO nanocomposite as catalyst (0.51 mol %) and sodium ascorbate as cocatalyst (1 mol %). KEYWORDS: Pristine graphene, RGO, Nanocomposites, CuAAC reactions

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INTRODUCTION Support materials in heterogeneous catalysts provide improvement in specific properties such as mechanical strength, stability, activity, and selectivity of catalysts. They also help in achieving uniform distribution of the particles of the active materials over a large area and reduce the chance of agglomeration of nanoparticles.1,2 In the past few decades, different carbonaceous nanomaterials such as mesoporous carbon, carbon nanotubes, and carbon nanofibers have shown great performance as catalyst support materials in numerous applications.3−5 The newest member of the carbon family, viz. graphene, has also showed its amazing capability as a nanomaterial support for catalysts.6,7 The atomically thin graphene sheets contain carbon atoms with sp2 hybridization that are arranged in a honeycomb like structure. This gives rise to very interesting properties and makes graphene an extraordinary material in demand. It has very high thermal conductivity,8 mobility of charge carriers,9 optical transmittance,10 Young’s modulus,11 and huge theoretical specific surface area.12 The combination of properties such as high surface area, extraordinary electronic transport properties, superior mechanical stiffness and flexibility makes it a superior conductive catalyst support for different kinds of nanoparticles.13,14 Additionally, the possibility of having π−π interaction helps graphene to adsorb organic reactants effectively and thus directly influence the chemical reactions. The π−π © 2017 American Chemical Society

stacking property of graphene is one of the dominant driving forces for the binding between the electron rich molecules and graphene sheets, leading to enhanced adsorption capacity of graphene, which helps in promoting the access of reactants to the catalytic metal nanoparticles making the reaction to proceed faster.15,16 A number of nanocomposites of metal or metal oxides with graphene have been synthesized.14 Reduced graphene oxide (RGO) has been the most predominantly used support for different nanocatalysts. However, even after every extent of reduction, RGO contains various residual functional groups attached to the graphene sheet leading to a support material with compromised properties.17,18 Almost eight atomic percent of oxygen atoms are present in RGO sheets.19 The presence of these sp3 sites disrupts the perfectly ordered sp2 arrangement of carbon atoms in the graphitic system. Presence of residual oxygen in graphitic system decreases the electron density, affecting the magnitude of the dispersion and exchange of πelectrons.20 Oxygen induced defects in sheets degrades the conductivity of graphitic system and also reduces its π−π interaction ability with other electron rich systems.21 These Received: March 29, 2017 Revised: June 19, 2017 Published: July 14, 2017 7632

DOI: 10.1021/acssuschemeng.7b00960 ACS Sustainable Chem. Eng. 2017, 5, 7632−7641

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ACS Sustainable Chemistry & Engineering

Figure 1. Characteristics of the GCuO nanocomposite: (a−d) TEM images showing the preferential deposition of CuO nanoparticles on G, (e) highresolution TEM image showing characteristic lattice fringes of CuO, (f) histogram showing particle size of CuO nanoparticles, (g) SAED patterns showing the lattice planes, and (h) SEM image showing preferential deposition of CuO nanoparticles over graphene sheet.

for making nanocomposites of G with CuO nanoparticles (GCuO). The nanocomposites showed superior catalytic activities in CuAAC reactions compared to the nanocomposite of RGO with CuO nanoparticles (RGOCuO). Wide applicability of triazoles in biological, industrial, agrochemical and optical applications prompted chemists to develop efficient and ecocompatible methods for their synthesis.23−26 Among the various methodologies, formation of 1,4disubstituted 1,2,3-triazoles using Cu(I)-catalyzed [3+ 2] cycloaddition of terminal alkynes and organic azides is the prime choice of the synthetic chemists.27−29 Various copper salts and complexes were used as homogeneous catalysts in organic

limitations of RGO as catalyst supports could be solved by the use of “pristine graphene” (G). The lack of simple methods for the preparation of G in the laboratory scale has been a challenge for its wider use as a catalyst support. Additionally, it has relatively low reactivity compared to RGO, thus making it a less popular candidate for the preparation of nanocomposites. With the introduction of liquid phase exfoliation methods, G can now be prepared in large quantities.22 Many studies have shown experimentally and theoretically that nanocomposites of G can be synthesized and have even better properties than the corresponding nanocomposites of RGO.13,19 The present paper describes a novel, green, and simple approach 7633


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Figure 2. (a) Raman spectrum of GCuO nanocomposite and (b) overlay of TGA thermal curves of the three samples that was recorded under an inert atmosphere. Preparation of G, RGO, CuO Nanoparticles and the Nanocomposites. Pristine graphene was synthesized by following our previously optimized procedure.13 RGO was prepared by following the modified Hummers method.27,37 Copper oxide nanoparticles (CuOnano) and the nanocomposites were prepared by microwave assisted hydrothermal route with some modifications.38 Details of all the experiments are given in the Supporting Information. Characterization. UV−visible absorption spectra of the synthesized nanomaterials after dispersing them in water were recorded using Shimadzu UV-4250 and PerkinElmer Lambda-750 spectrophotometers. Quartz cuvettes having path length of 1 cm were used in the UV−visible spectroscopic characterization. Thermogravimetric analysis (TGA) was done using Netzcsh STA F1 Jupiter to check the thermal stability of samples. Transmission electron microscopy (TEM) imaging and analysis was done using FEI Tecnai G2 200 S-Twin Electron microscope, operating at 200 keV. Raman spectra were recorded using RENISHAW Raman spectrometer with 532 nm laser at room temperature. X-ray diffraction (XRD) patterns were recorded in the 2θ range of 5−90° with Cu Kα radiation source (λ = 0.1542 nm, 40 mA, 45 kV) using RIGAKU diffractometer. The ICP-MS analysis of nanocomposites were done using Agilent 7900 instrument after digesting the samples in concentrated acid. Fourier transformed infrared (FTIR) spectra were recorded in the range of 400 to 4000 cm−1 using Carry-660 FT-IR spectrometer. 1H and 13C NMR spectra were recorded on Jeol JNM ECX 500 MHz spectrometer in CDCl3 or DMSO-d6. HRMS-ESI spectra were recorded on Bruker Maxis Impact HD instrument. High performance liquid chromatography (HPLC) was performed using Agilent RPC-18 column with dimensions of 4.6 × 150 mm. Injection volume used was 20 μL while a mixture of methanol: water (55:45) was used as the mobile phase. X-ray photoemission spectroscopy (XPS) was performed using PREVAC Scienta R3000 hemispherical analyzer.

solvents for performing CuAAC reactions. However, due to the difficulty in separation and purification of end products from homogeneous catalysts, heterogeneous copper catalysts have been developed for obtaining metal-free end products.30 Unfortunately, many heterogeneous catalysts also suffer from metal leaching into the reaction mixture often leading to contamination of the product.31 Development of catalysts that are capable of promoting the CuAAC reactions in water is also an open challenge as many catalysts lack stability and ability to promote the reaction in water.32 A number of solid supported copper nanocatalysts have been developed in recent years.27,28 Graphene based materials as supports for Cu catalysts have also been explored in CuAAC reactions as recyclable and reusable heterogeneous catalysts. However, mostly copper(I) oxide was used which is unstable and gets oxidized to Cu(II) oxide. Moreover, high amount of catalysts and extended reaction times (24−48 h) in a variety of organic solvent systems are to be used to achieve significant yields for most of the heterogeneous catalysts that are reported so far.33−36 Therefore, the development of an improved catalytic process especially for CuAAC using supported, recoverable heterogeneous catalysts would make it simpler, economically viable and ecofriendly. In this work we have conducted the CuAAC reactions in water which is a “green” medium using GCuO as a recyclable heterogeneous catalyst to achieve very good yields (88−98%) with high purity in shorter reaction time (1 h) at 30 °C leading to savings in energy. A small amount of catalyst (0.51 mol %) and cocatalyst (1 mol %) are only required to achieve very good yields. Also the stability of the catalyst was also observed by checking its recyclability (12 times) without showing any significant leaching of the metal to the reaction medium and loss of catalytic activity. Moreover, the exfoliation of graphite into graphene as well as the synthesis of GCuO was done using water as the medium without the use of corrosive chemicals. In contrast, the preparation of RGO involves the use of highly corrosive chemicals.

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RESULTS AND DISCUSSION Characterization of CuOnano and the Nanocomposites. The CuO nanoparticles and the nanocomposites were characterized by performing UV−visible absorption studies and the spectra are given in Figure S1a (Supporting Information). The spectra revealed the presence of both CuO and G in the nanocomposites.13 XRD patterns (Figure S1b) of all the three samples indicated toward the monoclinic symmetry of CuO.39 TEM images of CuOnano shown in Figure S2a and S2b revealed the formation of very small but agglomerated spherical CuO nanoparticles. A histogram showing the particle size distribution for the unsupported CuO particles is given in Figure S2c. The average particle size was 5.4 ± 1.4 nm. TEM images of the nanocomposite, RGO CuO are shown in Figure S3 (Supporting Information) where a preferential growth of spherical CuO nanoparticles was observed on RGO sheets. Histogram of particle size shown in Figure S3c revealed that

EXPERIMENTAL SECTION

Materials. Graphite powder was from Merck. Copper acetate monohydrate [Cu(CH3COO)2.H2O], sodium hydroxide (NaOH, 96% purity), acetic acid, ethanol, isopropanol, and ethylene glycol were purchased from Sigma-Aldrich. Chemicals for the synthesis of RGO and 1,2,3-triazole were also purchased from Sigma-Aldrich. All the chemicals were used as received without further purification. Ultrapure water from Milli-Q plus system (Millipore Co.) was exclusively used in all aqueous solutions and rinsing procedure. Whatmann Anodisc 25 filter of 20 nm pore size with the filtration setup was purchased from Millipore. 7634


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Figure 3. XPS spectrum of GCuO: (a) overall spectrum showing core levels of Cu, C, and O and (b) 2p core-level spectrum of Cu.

particles were uniform in size with average size of 4.4 ± 1.2 nm. Figure 1a−e shows the TEM images of GCuO. Spherical CuO particles were well separated and uniformly distributed on the pristine graphene sheets. Histogram shown in Figure 1f reveals the narrow particle size distribution. Average size of the particles was 4.3 ± 0.78. Figure 1e shows the HR-TEM image of particles on graphene sheets which shows the lattice fringe distances of 0.281 and 0.253 nm corresponding to the [110] and [111] planes of CuO.37 The selected area diffraction pattern (SAED) presented in Figure 1g revealed the polycrystalline nature of CuO nanoparticles in the GCuO nanocomposite. Several concentric rings with some bright diffraction spots have been found. SAED pattern also confirmed the presence of [110] and [111] planes in the nanocomposite supporting HR-TEM data shown in Figure 1e.38 FESEM image of GCuO is shown in Figure 1h, and it corresponded well to the TEM images. FESEM images of CuOnano and RGOCuO are shown in Figures S2d and S3d, respectively. While the spherical morphology of CuO nanoparticles was observed in both of these images, the particles were relatively agglomerated in CuOnano. Preferential deposition of well separated CuO nanoparticles was observed on both G and RGO sheets. Raman spectroscopy is a powerful technique for analyzing local atomic level arrangements and vibrations of the materials. It has been widely used to investigate the microstructural nature of different nanomaterials including graphene and CuO.39 Raman spectra in Figure 2a shows peaks related to both CuO and graphene in the nanocomposite. CuO nanoparticles in all the three samples belongs to the C62h space group and has three Raman active optical phonons (Ag + 2Bg).40 The observed Raman bands at 295, 335, and 615 cm−1 belong to the three well-known one-phonon modes of CuO.40 In addition to these peaks, a broadened peak at 1130 cm−1 corresponding to multiphonon (MP) transition was also observed for all the three samples.41 The Raman band that is indicated as Bg band is due to the stretching vibration in the x2−y2 plane of CuO.42 Intense peaks at around 1354 and 1625 and a broad peak at 2700 cm−1 were observed in the two nanocomposites corresponding to D, D′, and 2D bands of graphene.43 These peaks were absent for CuOnano (see Figure S2e and Figure S3e). The thermal stability of the prepared nanomaterials was studied by heating the sample from 20 to 800 °C using TGA and the thermal curves are shown in Figure 2b. The mass loss between 200 and 250 °C that was observed for CuOnano must be due to the decomposition of residual copper hydroxide to CuO.44 There was no further mass loss beyond 300 °C for CuOnano. The mass loss beyond 300 °C that was observed for the

GCuO nanocomposites is due to the decomposition of the graphitic backbone. Minor mass loss in the temperature range of 90 to 150 °C for RGOCuO must be due to loss of adsorbed solvent molecules. RGO has much lower thermal stability than G and hence it decomposes in two steps. The residual functional groups on the graphitic backbone decompose in the temperature range of 300 to 400 °C. Subsequently, complete decomposition of the graphitic backbone takes place slowly between 400 and 600 °C. However, decomposition of the graphitic backbone in G starts at only around 550 °C and completes at around 750 °C. The mass of the residue gives an approximate estimate of CuO in the nanocomposites. Accordingly, the mass of CuO was 80 and 74% by mass for RGOCuO and GCuO respectively from the dry mass of the nanocomposites, i.e. after correcting the moisture content. The surface chemical compositions for all the three products were characterized by XPS technique. Figure 3 shows the XPS spectra of GCuO nanocomposite. Figure 3a shows the overall spectra for GCuO nanocomposite revealing two peaks for O 1s and C 1s observed at 285.5 and 531.0 eV respectively.45 Two other peaks at 933.3 and 77.0 eV were also observed in both the spectrum of the composites, which are associated with Cu 2p and Cu 3p, respectively.45 High resolution spectra for Cu 2p (Figure 3b) exhibits a copper 2p1/2 peak at 955.0 eV and 2p3/2 peak at 935.0 eV, which are 1.3 and 0.2 eV higher than the peak positions for Cu(0) metal and are attributed to Cu(II).46 Two strong shakeup satellite peaks observed at 942.0 and at 962.1 eV confirm the presence of Cu(II) on the surface.47 The shakeup satellite peaks are evident and diagnostic of an open 3d9 shell. These observations confirm the Cu(+2) oxidation state and hence the formation of CuO. The gap between the Cu 2p3/2 and 2p1/2 is 20.0 eV, which is in agreement with the standard value of 20 eV for CuO.47 The strong peak at 935.0 eV belongs to the CuO, although it is higher than the reported energy of CuO.48,49 High resolution XPS-spectra for Cu 2p for CuOnano and RGOCuO are shown in Figure S2f and S3f. Both the spectra exhibited similar features as that for GCuO confirming the presence of Cu(II) on the surface. The amount of CuO in the nanocomposites was 82 and 76% by mass for RGOCuO and GCuO respectively as per the ICP-MS data. This data is more or less in agreement with the TGA data. Catalytic Performance of CuO/Graphene Nanocomposites. The aforementioned promising characterizations of newly synthesized nanocomposites encouraged us to evaluate their catalytic activity in the CuAAC reaction. A comparison of the catalytic activities of the CuO nanoparticles and the nanocomposites is given in Table 1. It can be clearly seen that GCuO was the best among the three catalysts. Further optimization of 7635


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two reactions were performed: with or without a reducing agent using a mixture of solvents (H2O + THF + t-BuOH). While the reaction completed within 1 h in the presence of the reducing agent, no progress was observed in the reaction without reducing agent. This is because Cu(I) is known to be the active form of catalyst in CuAAC reactions.50 Hence, a reducing agent is required to generate Cu(I) by the insitu reduction of CuO. Further, the same reaction was carried out in a mixture of H2O, THF as well as t-butanol. Amazingly the reaction could be completed with similar results. This has tempted us to check the efficiency of the catalyst in pure water. It was very exciting to perceive that in pure water also the catalyst displayed its catalytic efficiency in a similar fashion like the previous reaction carried out in mixture of solvents (See Table S1). Moreover, to optimize the amount of the catalyst, we performed a set of reactions with varying amounts of GCuO (0.51−5.1 mol %). The data is given in Table S2. Astonishingly, the catalyst was found to be efficient

Table 1. Comparison of Catalytic Activities of Different Catalysts for CuAAC Reactiona

catalyst

yield (%)b

CuOnano RGOCuO GCuO

50 86 99

a

1-Azido-4-nitrobenzene (1 mmol), phenyl acetylene (1.2 mmol), and sodium ascorbate (40 mol %), in water (5 mL) stirred for 1 h at 30 °C. b Isolated products.

reaction conditions was performed with GCuO as the catalyst to achieve better efficiency and ecofriendliness. In order to study the effect of presence of the reducing agent (sodium ascorbate),

Table 2. Synthesis of Different 1,4-Disubstituted 1,2,3-Triazoles at Optimized Reaction Conditionsa

a b

Reaction conditions: Azide (1 mmol), alkyne (1.2 mmol), sod ascorbate (1 mol %), and GCuO (0.51 mol %), in H2O stirred at 30 °C for 1 h. Isolated products. 7636


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Figure 4. TEM images of GCuO catalyst before and after the 12th cycle of reaction.

Table 3. Recyclability and Reusability of GCuO Catalysta cycle no.

1st

2nd

3rd

4th

5th

6th

7th

8th

9th

10th

11th

12th

recovered catalyst (%) yield (%) leached Cu (ppm)b

100 92 0.4

98 92

98 91

97 92 1.1

97 90

96 90

92 90

92 92 2.7

91 91

91 90

89 89

89 87 5.7

a

1-Azido-4-nitrobenzene (1 mmol) and phenyl acetylene (1.2 mmol) carried out under optimized reaction conditions. bThe entire reaction mixture was acidified and subjected directly to ICP-MS analysis.

Figure 5. Data for the reaction between 1-azido-4-nitrobenzene (1 mmol) and phenyl acetylene (1.2 mmol) using GCuO (0.51 mol %) as catalyst and ascorbic acid (1 mol %) as cocatalyst at 30 °C in water: (a) progress after separating the GCuO catalyst after 10 min (black squares) and 40 min (red dots) and (b) variation of yield (%) after 1 h wrt increase in reaction temperature.

Recyclability and Reusability. Recyclability and reusability is the most requisite parameter for the establishment of the admirable efficacy and the promising candidature of a catalyst. The CuAAC reaction was carried out using GCuO as catalyst, under the optimized conditions and the catalyst was recovered. After subsequent washing with water (three times) pure catalyst was obtained employing centrifugation technique followed by drying at 100 °C in a hot air oven. Complete recovery of the catalyst was not possible as a small amount was lost during repeated washing especially with the very small quantity of the catalyst (∼1 mg) used. However, the reclaimed catalyst was reused 12 times without any noticeable change in its catalytic activity (see Figure 4 and Table 3). Further, to check the alteration in the morphology of recycled catalyst, TEM images were acquired which reflected the unchanged morphology of the catalyst even after 12th cycle (see Figure 4). Checking the recyclability at conditions favoring high yields can often hide the deactivation of the catalysts. Hence, the recyclability was checked at conditions yielding partial conversion and the data is shown in

(95% yield) even in its minimum quantity (0.51 mol %). Hence, this amount of the catalyst was considered as its minimum quantity for a successful 1,3-dipolar cycloaddition reaction with excellent yield and high purity. The amount of reducing agent (sodium ascorbate) was also optimized and the data is shown in Table S2. It was found that only 1% (by moles) of the reducing agent was sufficient for the efficient activation of the catalyst. Further, to establish the catalytic efficacy of the catalyst under optimized conditions, several reactions were carried out by using different substrates under the optimized reaction conditions in water (See Table 2). The reactions with substrates having sensitive protective groups were also carried out. The successful completion of these reactions strongly reflected the efficient and promising candidature of GCuO nanocomposite. The isolated products were recrystallized with ethanol and characterized directly without column chromatography using various spectroscopic techniques such as FT-IR, 1H/13C NMR, and HRMS (see the Supporting Information). 7637


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Table 4. Some Recent Efficient CuAAC Reactions in the Presence of Various Catalysts and Reaction Conditions sl no 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

catalysts

solvent

amount (mol %)

time (h)

a. Triazoles Prepared from 1-Azido-4-nitrobenzene and Phenyl Acetylene chit-CuSO4 H2O 5 mg 6 3D graphene/Cu nanocomposites H2O 5 1 Cu1-USY toluene 10 15 gel-C60-CuCl2 DMSO 16 200 min Cu(PPh3)2NO3 solvent free 0.5 2 LCu(Cl)L solvent free 2 2 GCuO nanocomposite H2O 0.51 1 b. Triazoles Prepared from Various Substrates Cu@Fe NPs H2O 5 12 CuNPs CH2Cl2: t-BuOH 20% (w/w) 24 Cu-MONPs H2O 5−30 ppm 24 CuO-600 H2O: t-BuOH (2:1) 5 12 Cu@Fe3O4 DCM 1.54 12 CuNPs THF 10 10−120 min CuOx@Nb2O5 THF 1.2 6 CuOx@TiO2 THF 1−1.2 6 Cu(hexabenzyltren)Br H2O 4−200 ppm 24 Cu2O-NPs H2O 0.94 24 Cu/Cu-oxide NPs toluene 13−20% (w/w) 2−4 Cu NPs H2O 10 12 CNT-Ima-Cu(I) THF 2 96 CNT-Ima-Cu(I) THF: MeOH (30:1) 8 24 CRGO-Ima-Cu(I) deutrated THF 2 80 TRGO/Cu THF 2 48 PEG-tris-trz-CuI H2O 20−200 ppm 20 CuSO4/Ru−Mn complex EtOH 50/5 4.5−8 GCuO nanocomposite H2O 0.51 1

Figure S11. The reactions were carried out at 30 °C and stopped after 10 min. Product yield of 19.4% was obtained after the first cycle and only showed minor variations even after six cycles. Hence, the admirable recyclability and reusability without any morphological change of GCuO enlightened us about its outstanding stability. As the CuO is converted to Cu(I) by the reducing agent (sodium ascorbate), often the copper leaches from the solid catalyst in CuAAC reactions.51 The reaction proceeds practically under homogeneous conditions rather than heterogeneous catalysis when the leaching occurs. Hot filtration method followed by complementary ICP-MS analysis was performed to study the copper leaching in the present work.52 The CuAAC reaction between 1-azido-4-nitrobenzene and phenyl acetylene was performed using GCuO as the catalyst in two batches at 30 °C. The reaction was arrested after 10 and 40 min respectively in the above two batches by separating the solid catalysts and the precipitated triazole by centrifugation for 10 min followed by filtration. The reaction mixtures were kept stirring at 30 °C and further product formation was monitored at regular intervals using HPLC (Figure S12), and the data is reported in Figure 5a. The average yield was found to be 18 ± 3% after 10 min and 74 ± 2% after 40 min. Interestingly, no further product formation was observed after separating the solid catalysts. This indicates that reactions were mainly carried out by the heterogeneous catalyst in use and very less or no leaching of Cu ions takes place from the catalyst. The hot filtration method was supported by the ICP-MS study of the reaction mixture. The presence of copper in the reaction mixture after catalyst separation was checked after the first, fourth, eighth, and

T (°C)

yield (%)

ref

RT 70 RT hv RT RT 30

96 82 traces 98 98 98 92

56 57 58 59 60 61 this work

RT dark 50 RT RT 65 RT RT 30 37 RT RT 40 40 40

88−93 93 83−98 88−99 76−96 78−98 ND >99 ND >99 81−99 68−92 87−95 65 30 60 85−99 80−90 66−94 75−97 88−98

62 63 64 65 66 27 51 51 67 68 69 70 71 71 71 72 73 74 this work

35 hv 30

twelveth cycles during the recyclability testing. The results are reported in Table 3. Evidently, there was only 0.4 ppm copper content after the first cycle and the amount of copper increased during subsequent recycling. However, the maximum amount of copper leached was only 5.7 ppm after the 12th cycle. This is well within the internationally accepted residual copper content (15 ppm) for pharmaceutical applications.53−55 The hot filtration experiment was further supported by complementary results using 5.7 ppm of CuI as homogeneous catalyst after confirming the +1 oxidation state of Cu using XPS (Figure S14a). There was no appreciable progress in the reaction within 1 h from the HPLC data given in Figure S14b. The homogeneous catalysis versus heterogeneous catalysis question was further probed by analyzing the intermediate that was formed in the solid state when the reaction was performed in absence of the azide. The GCuO that was brownish black in color got converted into a greenish yellow colored solid (Figure S15) that was separated and purified. FTIR (Figure S16) and XPS (Figure S17) data confirmed the formation of Cu(I)-phenylacetylide. Reaction temperature is a very important factor in deciding the rates and yields in catalytic reactions. The product yield during the synthesis of triazoles can be tailored with different temperature conditions. In order to see the effect of temperature, we performed the reaction at the temperatures ranging from 25 to 50 °C. The variation of yield (%) after 1 h under different reaction temperatures are shown in Figure 5b. Complete conversation of the reaction was possible at 45 °C in 1 h. Activity of nanocatalysts depends very much on the surface area. Increased activity of supported catalysts (GCuO and RGOCuO) compared to unsupported one (CuOnano) could easily 7638


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ACS Sustainable Chemistry & Engineering be explained on the basis of less agglomeration and hence the availability of high surface area of CuO nanoparticles. Interestingly, among the two supported catalysts, total surface area of CuO nanoparticles on RGO should be higher. This is because, the particle size of CuO nanoparticles on both G and RGO was same while the amount of CuO on RGO was more (82%) than on G (76%). The better catalytic activity of GCuO than RGOCuO must be then due to the support material. Graphene support must be playing a very crucial role in adsorbing the reactants through the π−π interaction and showing better conductivity compared to RGO support. The catalytic activity of GCuO was compared with other copper based catalysts that are reported in the literature. Although, a meaningful comparison of catalytic activities is extremely difficult due to the variability of reactants, a compilation of some of the recently reported copper based catalysts and their catalytic performance is given in Table 4. The table lists six papers where the CuAAc reaction between 1-azido-4-nitrobenzene and phenyl acetylene was performed in the presence of copper based homogeneous and heterogeneous catalysts. Evidently, GCuO outperformed all the reported catalysts for this reaction. A large number of other heterogeneous catalysts that are reported in the literature are also summarized in Table 4. It is amply clear from the table that a combination of higher amount of catalyst and cocatalyst, solvents other than water, higher temperature, and longer reaction time is required to achieve comparable yields using catalysts that are reported in the literature. However, only 0.51 mol % of GCuO along with 1 mol % of cocatalyst (sodium ascorbate) is sufficient to achieve more than 90% yield at 30 °C in water.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.F.S.). *E-mail: [email protected] (S.G.). ORCID

Rituporn Gogoi: 0000-0003-4652-4013 Subrata Ghosh: 0000-0002-8030-4519 Prem Felix Siril: 0000-0002-8818-7310 Author Contributions ‡

T.V. and R.G. contributed equally.

Funding

Funding from DST, Government of India, through SR/FT/CS56 (2010G) is acknowledged. T.V. and R.G. acknowledge a HTRA fellowship from MHRD, and P.G. is thankful to DRDO for research fellowships. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Advanced Materials Research Centre, IIT Mandi, is acknowledged here for infrastructure facilities. Assistance from Mr. Midathala Yogesh in HPLC analysis is also gratefully acknowledged.

CONCLUSIONS We have studied the effect of presence of different types of graphene supports (G versus RGO) on the morphology of CuO in their respective nanocomposites and the catalytic activities of these nanocomposites in CuAAC reaction. Defect free pristine graphene was found to be a better catalyst support material than RGO. Pristine graphene can be synthesized in large quantities without using any corrosive chemicals unlike for the synthesis of RGO where a number of highly corrosive chemicals have to be used. Both RGO and G act as very good support materials by preventing the aggregation of the CuO nanoparticles. Better catalytic activity of the nanocomposite of pristine graphene over the RGO must be due to its better conductivity and ability to adsorb reactants through π−π interaction. The better adsorption ability of pristine graphene must also be one of the reasons for the ability of CuO nanoparticles to promote the reaction without significant leaching of the metal. The GCuO catalyst promotes the CuAAC reaction by acting as a true heterogeneous catalyst by adsorbing the reactants on the particle surface and formation of Cu(I)−phenylacetylide intermediate in the solid state. The present work opens up the possibility to develop highly active catalysts using pristine graphene as a catalyst support.

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and RGOCuO; catalytic efficiency of CuOnano, RGOCuO, and GCuO in CuAAC reactions; NMR and HRMS data of different molecules synthesized using GCuO catalyst in CuAAC reactions; HPLC data following the progress of reaction after hot filtration and using CuI as a homogeneous catalyst; FTIR and XPS data showing the formation of the Cu(I)−phenylacetylide intermediate in the solid state (PDF)

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00960. Detailed characterization of CuOnano and RGO nanocomposites; TEM, SEM, Raman, XPS analysis, and histograms depicting particle size distribution of CuOnano 7639


Research Article


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Pristine graphene - copper (II) oxide nano catalyst: A novel and green

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